1. Field of the Invention
The present invention generally relates to electrodes and more particularly relates to an improved method of making a cadmium electrode
2. Prior Art
Electrodes utilized in long-lived alkaline nickel-cadmium batteries desirably exhibit high densities and high percentage of active material. The cadmium electrode which is used as the negative electrode in such batteries is formed by impregnating a porous sintered nickel body known as a plaque with active cadmium material, that is hydroxide, or sometimes the oxide. The most widely used deposition method consists of filling the pores of the plaque with a solution of cadmium-bearing salts or with a molten cadmium salt, usually with the aid of a vacuum. The cadmium-bearing salt, once deposited in the pores or cavities of the plaque, is then converted to cadmium hydroxide by treatment with an alkaline solution. This is often accompanied by cathodic polarization to enhance the migration of the alkali into the plaque. This impregnation procedure has to be repeated several times in order to increase the concentration of cadmium hydroxide to an acceptable level.
Molten cadmium nitrate is often used (Cd(NO.sub.3).sub.2.4H.sub.2 O because it is the most highly concentrated source of cadmium ions, with a density of 2.03 grams per cubic centimeter just above its melting point and with a cadmium ion content of 36.4%. This results in a loading level of the cadmium hydroxide of 1.65 grams per cubic centimeter of void volume in the porous plaque. A second impregnation cycle with the molten cadmium nitrate theoretically would increase the loading level to 2.73 grams per centimeter of void volume. However, penetration of the molten salt is less than complete becuase of the high viscosity of the liquid, the vapor pressure of the liquid and the capillary size of some of the voids. Void penetration is usually only about 65% effective. In order to reach loading levels in excess of about 2.4 grams per cubic centimeter of void volume, this procedure requires from about 3 to 9 impregnation cycles, and thus is quite expensive and time consuming.
More recently, electrochemical impregnation methods have been developed to achieve suitably high impregnation levels in a single step. Cadmium-bearing salt solutions are used, generally of about 2-3 M concentration at about 85.degree.-105.degree. C. Cadmium nitrate is usually used at about 2-5 pH, the acidity being maintained by periodic addition of nitric acid. An older process along the same lines utilizes more dilute solutions and room temperature, as specified in U.S. Pat. No. 3,214,355, but the loading levels achieved are too low to be significant. U.S. Pat. No. 3,573,101 follows the teachings of U.S. Pat. No. 3,507,699 but applies both elevated temperature and potassium nitrite, the latter to stabilize the pH. A slightly different procedure is set forth in U.S. Pat. No. 3,873,368 but in all these cases of electrochemical impregnation, the loading level of cadmium hydroxide in the plaque remains significantly below 2.4 grams and usually is not above 1.8 grams per cubic centimeter of void volume in the plaque.
Therefore, it would be desirable to provide a simple, inexpensive, highly efficient method of impregnation of cadmium hydroxide in porous sintered nickel plaque, preferably by a single step method which would result in a loading level well above 2 grams per cubic centimeter of void volume without having to repeat the process.